Power converters are used to convert AC utility power for use by load devices requiring a reduced or boosted AC or DC supply voltage and/or by load devices that require an AC supply voltage at frequencies that are different from the conventional 50/60 Hz utility grid supply. Conventional utility power converters often include a utility-side converter connected to the utility grid through a contactor and filter circuits. The output of the utility-side converter is typically rectified, boosted, and supplied to a DC bus, including charging one or more DC bus capacitors. The DC bus is then linked to the inputs of one or more load-side converters, the outputs of which are connected to a load device such as an AC or DC motor/generator or a battery or other energy storage system.
The power converter usually includes a control circuit containing control logic that initiates and regulates operation of the converter. The control circuit requires its own control power supply so that the control circuit can perform its start-up and initial control functions independent of the operation of the converter. In conventional utility grid power converters, a source of external power therefore must be connected to the converter to energize the internal control circuit power supply. This is often accomplished through the use of a battery or other power source that is separate from the utility grid.
In addition, a utility power converter includes a pre-charge circuit to pre-charge the DC bus capacitor(s) before the converter is activated. A conventional pre-charge circuit often uses a resistor network connected in parallel with the power line contactor, utilizing the diodes inherent in the utility-side converter to rectify the grid power into the DC bus. Although the resistors are effective in limiting the in-rush current to the DC bus capacitor(s), they introduce energy losses from resistive heating. The resistors are subject to overheating and failure in the event of an electrical fault in the contactor circuit, and/or a fault in a converter on the DC bus. Also, because the resistor network electrically connects the contactor to the DC bus, there is no galvanic isolation between the power grid and the output of the converter.
What is needed is a pre-charge system and method for a power converter without the disadvantages described above.